Negima!'s consistent, and impressive art has a good balance of detail, and simpleness.

Katekyo Hitman REBORN!

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REBORN!'s art style is a bit sketchy at times, but it becomes really impressive when combined with alot of detail & heavy amounts of ink..

Claymore

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Claymore's large amount of detail, and HUGE amount of creativity looks amazing.

Medaka Box

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Medaka Box's consistent artstyle makes great use of various inks, and tones. This, along with REALLY good detail throught most of the series makes Medaka Box's art really stand out.

Bakuman

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Bakuman's detailed art can look comical at times, but it really does an amazing job at expressing a characters emotions, and attitude. It may not be as visually impressive as others, but it's very powerful.

1. Rosario + Vampire Season II
In my opinion the best looking and detailed Shounen Manga in the market as of now. It is a Monthly Release so Mangaka Akihisa has a lot more time to put in detail and beautiful drawings. You can see the progress he made between the beginning of Season I to the current chapters of Season II.

2. Akame ga Kiru!
Same as above but with different Mangaka.

3. Great Teacher Onizuka
One of my all time favorite manga series not only for its great characters and plot, but also for its great art. Despite being a weekly release manga, it manages to pull off the look of a monthly release manga.

4. Kaichou wa Maid-Sama!
Best looking Shoujo Manga in the market as of now. The characters are much more realistically drawn when compared to the vast majority of other Shoujo Mangas. The art seems to have some aspects of Shounen in it.

5. Until Death Do us Part
Great Seinen Manga with great plot and art.

6. Medaka Box
Definitely not the most detailed nor the most artistic style, but for a weekly shounen Jump manga (same magazine as Naruto) it looks really good. Definitely much better than Naruto or Bleach.